When a load current flows from an AC power supply to a load, so-called harmonic components are typically generated in the load current. Reducing the harmonic components, which cause so-called harmonic interference, is a well-known subject.
As an approach of meeting the subject, an active filter is proposed. The active filter operates so as to prevent the harmonic components of the load current from flowing into the AC power supply.
For example, a parallel active filter is connected to the AC power supply through an interconnection reactor. Passing a compensating current from the parallel active filter reduces the harmonic components of a power current flowing through the AC power supply.
For the parallel active filter to function as described above, a control input for controlling the parallel active filter is required.
The parallel active filter is connected to the AC power supply together with the load as described above. Thus, the parallel active filter including the interconnection reactor and the load are considered collectively to be a to-be-controlled system.
The parallel active filter operates based on a control input, and accordingly, the to-be-controlled system is also considered to operate based on the control input.
The compensating current varies in accordance with the power phase of the AC power supply. In the to-be-controlled system, an observable amount varying cyclically, namely, a compensating current is accordingly obtained.
The control input is determined based on a deviation between a command value and a detection value (hereinafter referred to as a “compensating current command value” and a “compensating current detection value,” respectively) of the compensating current.
Therefore, the to-be-controlled system can be generalized and considered as follows: it operates based on a control input determined based on an observable amount varying cyclically.
As described above, the control input of the parallel active filter is based on a deviation between the compensating current command value and the compensating current detection value. More specifically, a control input is determined as a sum of a constant multiple of an integrated value of a deviation and a constant multiple obtained as a result of the accumulation of a deviation in accordance with the power phase.